xvp is a web interface for XenServer, developed by Colin Dean at Durham University in the UK.
There are four components:
The whole thing is open source (GPL v2), using libxenserver, XML-RPC for PHP, and parts of TightVNC. It's been in development since May 2009.
Project home page: http://www.xvpsource.org/
xvpweb on Windows
xvpviewer on Mac OS X
xvpviewer on Linux